This invention relates to localisation and mapping. In some computer vision applications, there is a requirement to be able to process the 3-D position of image features captured by a 2-D camera. One example situation is found in robotics, in that a robot using computer vision may need to be able to map its environment and also know its own location with respect to that environment. Another example situation occurs in videogames, in that, for example, a hand-held gaming device having a camera built into the device can be used to capture images of the real surroundings, onto which so-called augmented reality (AR) image features can be rendered. For example, a gaming device may capture an image of a real building, but this is displayed to the user with an animal, superhero or other image rendered so as to be climbing up the side of the building. In another example, a computer-generated creature may be rendered so as to appear (in the rendered image) on a real table top.